Phosphorus chemistry plays a central role in cellular metabolism and the study of phosphorus esters and chloro acids has provided information pertinent to the understanding of enzyme-catalyzed phosphoryl transfer reactions. Although the kinetics of hydrolysis of phosphorus pentachloride(1) and phosphorus oxychloride (2) have been studied under pseudo first order conditions a method for analyzing the composition of partial hydrolysis mixtures formed in situ has been lacking even though (2) plus water is an important phosphorylating agent for the synthesis of a variety of biological compounds. A procedure for analyzing the compositions of such mixtures has been developed using partial hydrolysis of (1) in highly-enriched 18-O water followed by complete hydrolysis in normal enrichment water. GC-MS analysis of the trimethyl phosphate isotopomers of such mixtures yields spectra that can be analysed by linear regression analysis to provide accurate estimates of the components which include in addition to (2), the di- (3) and mono- (4) phosphorochloridic acids as well as orthophosphoric acid (5). The hitherto undetected monochloro acid (4) was found in yields up to 30% in a narrow range of hydrolysis -- an observation that may be relevant to the use of (2) as a phosphorylating agent. Its appearance is attributed to a change in its hydrolytic mechanism from a fast dissociative process in an excess of water to a slower associative process in limiting water. The in situ hydrolysis reaction has been modelled kinetically. Relative values were obtained for the four second order rate constants using a combination of numerical integration and a minimization procedure that accurately described the end point compositions of mixtures over a wide range of hydrolysis. The procedure used may have a general applicability to other phosphorus halides and oxyhalides and may have practical relevance to chemical synthesis of biologicals and to active site considerations in phosphotransfer reactions.